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Friday, March 18, 2011

The Temple of the Mind

At least one source says that the Temporal Lobe is so named because the forward edge lies just below the temple. More likely it refers to proximity to the temporal bone, just below the temporal and forward of the ear. As you may recall, in the naming exercise for lobes of the brain, I referred to the Temporal Lobe as the *thumb* of a mitten or baseball glove. Like a glove, the inside of the Temporal Lobe is as important as the outside.

We have come to realize that the only lobe which has only one function in the human brain is the Occipital Lobe (and that is due to the dominant role of vision in primates). Like the Parietal Lobe, the Temporal Lobe fulfills many duties: sensory (hearing), executive (memory) and association (auditory and memory). It should be noted that these divisions of the brain are largely due to the surface landmarks, and bear much less relationship to function. If pure function were invoked, there would be many more than just five lobes to the brain (and in fact, some catalogs list a “Limbic Lobe” which resides entirely within the inner surface between hemispheres and on the inner Temporal Lobe). The Dorsal (upper) surface of the Temporal Lobe contains auditory cortex and association areas. The lateral surface actually contains some of the memory storage of the brain, and is responsible for names and proper nouns (which makes sense in proximity to hearing). The ventral (lower) regions contain entorhinal cortex – one of the first relays in the memory system. Inner surface contains the hippocampus (see diagram, right) which is the primary relay for formation of new memory.

One of the terms most frequently associated with this region is “rhinal” (nose) named for the rhinal fissure, one of the deep sulci or grooves in the lower surface of the Temporal Lobe. The surrounding areas of Temporal Lobe contain the entorhinal and perirhinal cortex. On the medial (inner) surface is hippocampus and parahippocampal areas. Together, these areas serve as coordination and association areas for memory. In fact, most memory storage in the brain is via association. Related facts are stored together, so that recall of one memory can trigger recall of another.

Another feature of the Temporal Lobe is that the hippocampus is the terminal point of a circuit termed the “Limbic System” which, along with the amygdala, is commonly mis-stated as being responsible for emotion. Rather than being the embodiment of emotion (that role is reserved for Frontal and Prefrontal areas) the amygdala is involved in evaluating emotional content of events – and particularly memory. However, those are topics for the next blog post on hippocampus and limbic system.

If “newness” (in terms of development) of brain areas is measured by the degree of folding and convolution, then Temporal Lobe is among the “newest” areas of the primate brain. Yet the hippocampus and limbic system are some of the “oldest” structures in the brain, dating back to rudimentary brains of lizards and birds. More likely, the Temporal Lobe is an old structure with new functions added on as the brain developed. In some ways it is most simplified, and in others, the most complex. For this reason we will spend several days on the structures and functions as we explore the implications of memory in the mammalian brain.